1. Field of the Invention
The present invention relates to alight emitting device and a method for manufacturing the light emitting device. Particularly, the present invention relates to a light emitting device having a high output power and reliability, which is used for an illumination apparatus, a backlight, a light emitting apparatus mounted on a vehicle, a display, an auxiliary light source for moving image illumination, and other general industrial and consumer light sources, and a method for manufacturing the light emitting device.
2. Description of Related Art
A light emitting device using a light emitting element such as a light emitting diode (LED) and a laser diode (LD), and a method for manufacturing the light emitting device have been developed recently. For example, such light emitting elements and manufacturing methods are disclosed in Japanese Laid-Open Patent Nos. 2001-177160, 2005-294736, H11-45958, and 2002-520823. FIGS. 12A and 12B show diagrams which indicate a construction of a conventional light emitting device. FIG. 12A is a schematic plan view of a conventional light emitting device observed from a main light emitting surface side, and FIG. 12B is an XII-XII cross-sectional view of FIG. 12A. A conventional light emitting device 301, as shown in FIGS. 12A and 12B, includes a light emitting element 310, lead frames 320 and 330, a casing (package) 340 that is a resin forming body to cover almost parts of the respective lead frames 320 and 330, and a covering member (sealing member) 350 that is a resin forming body covering the light emitting element 310. The casing 340 is composed of a thermoplastic resin based composite material and has a recess in the center of the casing 340. From a bottom of the recess, an inner lead 320a which is a part of a main surface side of the lead frame 330, is exposed, and the light emitting element 310 is mounted on the exposed part. The inner lead 320a is connected to the light emitting element 310 through a wire (conducting wire) 360a. Further, from the bottom of the recess of the casing 340, an inner lead 330a which is a part of the main surface side of the lead frame 330, is exposed, and the inner lead 330a is connected to the light emitting element 310 through a wire (conducting wire) 360b. The covering member 350 is composed of a good translucent member such as a thermosetting type silicon resin. The light emitting device 301 has a structure as mentioned below in order to efficiently reflect light radiated by the light emitting element 310. That is, the inner leads 320a and 330a are constructed so that an area of the main surface side is as large as possible, and a side surface inside the recess of the casing 340 is constructed to have a conical shape so that the side surface has an inclination widening upward.
A technology described in Japanese Laid-Open Patent No. 2001-177160 is directed to an object to prevent lifting of the lead frames 320 and 330 which is caused by a thermal expansion of the covering 550 in a reflow process. A thermoplastic resin based composite material such as nylon that is generally used for a material of the casing 340 of the light emitting element 310 becomes soft when it is heated. Hereby, a problem that the lead frames 320 and 330 lift is caused. On the contrary, if a thermosetting resin based composite material such as an epoxy resin is used for a material of the casing 340, the lead frames 340 and 340 hardly lift since a thermal expansion coefficient of the covering member 350 is close to that of the casing 340. Herein, Japanese Laid-Open Patent Nos. 2005-294736 and H11-45958 disclose general manufacturing methods for the conventional light emitting device 301. Further, Japanese Laid-Open Patent No. 2002-520823 discloses a technology that the inner leads 320a and 330a are covered at the bottom surface of the recess of the casing 340 excluding regions needed to electrically connect the light emitting element 310 by using a thermoplastic resin such as polycarbonate, and the bottom surface and the side wall of the recess function as a reflector.
In general, fluidity of the thermoplastic resin based composite material is inferior to that of the thermosetting resin based composite material. Therefore, it is difficult to form a molding in a thin shape for the thermoplastic resin based composite material. When one wants to obtain a molding by using the thermoplastic resin based composite material and achieving a high fluidity needed for molding, it is necessary to increase a molding temperature and a mold temperature to high. However, in this case, it is impossible to obtain a sufficient strength of the molding needed for releasing in an injection molding process. Hereby, there is a case that chipping of the molding occurs when the molding is taken out from molds. Further, if the molding temperature and the mold temperature are high, there is a case that a resin becomes deteriorating due to the high temperature and a color of the molding changes. Meanwhile, by reducing a filling amount of a filling agent (filler) in the thermoplastic resin composite material, it may be possible to achieve a high fluidity needed for a molding process. However, if a filling amount of filler in the light emitting device is reduced in the same way, for example, a reflection rate of the resin is decreased due to a decrease in an amount of a reflection substance, resulting in a significant reduction of performance of the light emitting device. On the other hand, it is possible to fill a filling agent at a high concentration for a thermosetting resin based composite material. Additionally, the thermosetting resin based composite material has a high fluidity comparing to the thermoplastic resin based composite material. For this reason as mentioned above, it is proposed to use a thermosetting resin based composite material for a material of the casing 340 of the light emitting element 310, in the light emitting device 301. However, in a case when the light emitting device 301 is manufactured by using a thermoplastic resin based composite material, a manufacturing process thereof is simpler and a manufacturing cost is lower than a case when a thermosetting resin based composite material is used. For the above mentioned reason, a casing 340 produced by using a thermosetting resin based composite material has not been realized to date. Herein, in a view point of adhesiveness between the casing 340 and the covering member 350, a thermosetting resin based composite material may be more preferable for a casing 340 material than a thermoplastic resin based composite material. Further, in a view point of adhesiveness between the casing 340 and the lead frames 320 and 330, a thermosetting resin based composite material may be more preferable for a casing 340 material than a thermoplastic resin based composite material.
The light emitting element 301 is free from burnout because the light emitting element is a semiconductor element, having excellent initial drive performance, strength in vibration, and excellent durability for repeated ON/OFF operations. Owing to such excellent properties, the light emitting device is utilized for illumination outside. Meanwhile, in a long period usage, oxygen or sulfide comes from outside air into the covering 350 of the light emitting element 310, and changes the surfaces of the inner leads 320a and 330a of the respective lead frames 320 and 330. Further, light and heat generated by light emission of the light emitting element 310 also change the color of the surfaces of the inner leads 320a and 330a. If the surfaces of the inner leads 320a and 330a are plated, the color of the plating film changes to black. Accordingly, a reflection rate of the inner leads 320a and 330a surrounding the light emitting element 310 is decreased, which leads to a decrease in output power of the light emitting device 301 (namely, becoming dark). Due to the above mentioned reasons, methods for elongating the life of the light emitting device 301 are proposed, in which the inner leads 320a and 330a surrounding the light emitting element are coated by a resin or an inorganic material. However, according to the above mentioned methods, a coating process for the inner leads 320a and 330a surrounding the light emitting element 310 is newly and additionally needed in a manufacturing process of the light emitting device 301. Moreover, since a coated part of the inner leads 320a and 330a plays as an insulating film, the light emitting element 310 is not electrically connected to the inner leads when it is mounted after the coating process. Therefore, a time-consuming process is needed, that is, a process that a part of the coating region is needed to be peeled off after coating all of the surfaces of the inner leads 320a and 330a, or a process that only predetermined regions of the inner leads 320a and 330a are beforehand coated, is needed. This causes a problem of reducing a productivity because the manufacturing process becomes more complicated.
The present invention has been developed to solve the above mentioned problems. Therefore, it is an object of the present invention to provide a light emitting device that is capable of maintaining a stable light emitting property for a long period. Further, it is another object of the present invention to provide a manufacturing method to efficiently produce a light emitting device that is capable of maintaining a stable light emitting property for a long period.